A handful of Republican House members are among the largest contributors to a campaign to repeal California’s gas tax boost, one that could draw party voters to the polls in competitive congressional districts.

The seven lawmakers include House Speaker Paul D. Ryan of Wisconsin and second-term Rep. Mimi Walters of California. They have contributed or loaned more than $1 million of the $2.2 million received by a group called “Yes on 6, Repeal the Gas Tax,” according to second-quarter state election reports filed July 31.

The ballot initiative, known as Proposition 6, would reverse a 2017 state law that raised gasoline taxes by 12 cents per gallon and diesel taxes by 20 cents per gallon, with a vehicle registration fee, to pay for road repairs statewide and transit in the Los Angeles area.

With no competitive races at the top of the state ballot in an unfavorable midterm year for Republicans, the measure could boost turnout among otherwise disaffected GOP voters in a state vital to the party’s House majority. Larry Gerston, a political scientist at San Jose State University, said in a phone interview the ballot initiative allows Republicans to get out their voters to a greater degree than they would otherwise.

The difference in turnout could be a deciding factor for Walters’ and other close races, he added.

That motivation is reflected in part by the campaign’s biggest donors: Walters and national party leaders like Ryan and Majority Leader Kevin McCarthy.

Republican gubernatorial candidate John Cox also contributed.

The campaign committee for Walters and an associated PAC have loaned or contributed $339,000 to Yes on 6. McCarthy’s campaign has provided $300,000 and House Intelligence Chairman Devin Nunes of California has pitched in $100,000.

For Walters, supporting the measure serves two purposes, Gerston said.

“It’s a two-fer for her,” he said. “Yes, she pitches in to get the statewide vote up. But it also motivates her base in her own district because she can be out there saying ‘I’m leading this fight.’”

One of only three issues listed on Walters’ campaign website is a section on “Debt and Taxes.”

“Americans are taxed too much and those of us who live in California are hit even harder with high taxes,” her website says.

Walters is locked in a race rated Leans Republican by Inside Elections with Nathan L. Gonzales.

The campaign committee for Ryan, who is retiring, has also contributed $50,000 to the effort. Gerston said Ryan could be involved as a favor to McCarthy, whom he has endorsed to succeed him. If the effort succeeds in keeping vulnerable Republicans in swing seats, McCarthy could take credit, he said.

House Majority Whip Steve Scalise has also contributed $25,000.

One California Republican in a close race who has not backed the effort — at least financially — is Rep. Jeff Denham, who plans to run for the chairmanship of the House Transportation and Infrastructure Committee if he wins re-election. Retiring Chairman Bill Shuster has proposed raising federal fuel taxes, though Republican leaders oppose it.

A spokeswoman for Denham’s congressional office referred an inquiry to his campaign, which did not immediately return an email. Representatives for Walters and McCarthy did not reply to requests for comment.

Two other Republican lawmakers from California also contributed to the Yes on 6 effort: Rep. Ken Calvert’s campaign committee gave $125,000 and freshman Rep. Doug LaMalfa’s campaign contributed $12,500.